1. Field of the Invention
The present invention relates to an acoustic signal processing apparatus and an acoustic signal processing method for signal processing of an input acoustic signal to be reproduced by a loudspeaker.
2. Description of the Related Art
Down-sizing demands are on the increase in cellular phones and the like, and loudspeakers built in the devices are demanded to be down-sized. However, there is a problem that low pitched or low frequency sound is not sufficiently reproduced. To solve such a problem, in the small-sized loudspeakers, low-sound reproduction is performed by using a harmonic in place of low pitch sound which is difficult to be sufficiently reproduced. It is known as the missing fundamental phenomenon in which one hears a sound like a sound which includes a sound of a frequency even if the sound of the frequency is not included, when a sound including a harmonic of the frequency (a sound of integral multiple of a frequency) is heard. Therefore, by reproducing a low pitch sound harmonic in place of a low pitch sound, a low pitch sound which is not really reproduced is heard as if the low pitch sound is reproduced.
Patent document 1 (Japanese Patent Application Publication Kokai No. 2006-222670 discloses an apparatus which controls the directivity of sound reproduced by a loudspeaker, by using a low pitch sound harmonic in place of a low pitch sound which is difficult to control its directivity. In this apparatus, a fundamental frequency is detected, by submitting to fast Fourier transform, an acoustic signal reproduced by a loudspeaker. Then, the harmonic of the detected fundamental frequency is emphasized and reproduced by the loudspeaker.
Also, Patent document 2 (Japanese Patent Application Publication Kokai No. 2004-151225) discloses an apparatus which permits to make easily perceive a low pitch sound by adding to an acoustic signal an odd-numbered harmonic of the acoustic signal reproduced by a loudspeaker. In this apparatus, by submitting an acoustic signal reproduced by a loudspeaker to fast Fourier transform processing and modulation processing, an odd-numbered harmonic of the acoustic signal is generated. Then, the generated odd-numbered harmonic component is added to the acoustic signal and reproduced by the loudspeaker.
In the apparatus described above in the Patent Document 1, by using a harmonic in place of a low pitch sound of which directivity is difficult to be controlled, directivity of a sound reproduced by a loudspeaker is controlled. This apparatus is used for controlling the directivity of sound reproduced by a large-sized loudspeaker, and not applied to a small-sized loudspeaker which cannot reproduce low pitch sound sufficiently. Moreover, in the apparatus described in the Patent Document 2, by submitting an acoustic signal to fast Fourier transform processing, an odd-numbered harmonic is generated. Then, by adding the generated odd-numbered harmonic to the acoustic signal, it is possible to make easily perceive a low pitch sound. However, in this apparatus, it is impossible to increase the volume of a sound reproduced by a loudspeaker without increasing output electric power. Moreover, the apparatuses described in Patent Documents 1 and 2, are forced to have an enormous amount of processing, due to fast Fourier transform processing utilized.